1. Field of the Invention
The present invention relates to an image forming apparatus and a thermal transfer printer. More specifically, the present invention relates to an image forming apparatus and a thermal transfer printer equipped with a printing head for performing printing.
2. Background Information
An image forming apparatus equipped with a thermal head or other such printing head has been known. For example, Japanese Patent Application Publication 5-85012 discloses such arrangement.
Japanese Patent Application Publication 5-85012 discloses a head pressing apparatus for a thermal printer, which includes a torsion bar having ends that are bent at right angles in opposite directions. The torsion bar is disposed at the top of a thermal head, and the bent ends of the torsion bar are pressed during printing. Therefore, the pressing forces applied to the bent ends of the torsion bar are transmitted to the thermal head in a balanced manner through the torsion bar, which suppresses the difference in the pressing force in the width direction of the thermal head.
Also, thermal transfer printers are known as a type of image forming apparatus. FIGS. 40 and 41 are oblique views of the overall structure of a conventional thermal transfer printer. FIGS. 42 and 43 are oblique views illustrating the attachment structure of a pressing member and a shaft. FIG. 44 is a cross section illustrating the pressing operation of the thermal head on the platen roller in the conventional thermal transfer printer shown in FIGS. 40 and 41. The structure of a conventional thermal transfer printer will now be described through reference to FIGS. 40 to 44.
As shown in FIGS. 40 and 41, this conventional thermal transfer printer includes a metal frame 101, a thermal head 102 for performing printing, a platen roller 103, platen roller bearings 104 that rotatably support the platen roller 103, a metal shaft 105, pressing members 106 and 107, a drive gear 108 having a small-diameter gear 108a and a large-diameter gear 108b, both made of resin, for pivoting the pressing member 106, a torsion coil spring 109, a motor 110, a motor bracket 111, and an intermediate gear 112.
Also, as shown in FIGS. 40 and 41, an ink sheet insertion portion 101c for attaching an ink sheet (not shown) is provided to a second surface 101b, which is located opposite a first surface 101a of the frame 101 to which the motor bracket 111 is attached. Insertion holes 101d into which the ends of the shaft 105 are rotatably inserted are provided in the first surface 101a and the second surface 101b of the frame 101. The thermal head 102 is attached so as to be pivotable around a support shaft 102a on the inside of the two side surfaces of the frame 101. The torsion coil spring 109 is attached to the support shaft 102a of the thermal head 102. The function of this torsion coil spring 109 is to urge the thermal head 102 away from the platen roller 103. Also, a head component 102b provided to the lower part of the thermal head 102 is disposed opposite the platen roller 103. Bent components 102c that are pressed on by the pressing members 106 and 107 are formed above the ends of the head component 102b of the thermal head 102.
As shown in FIGS. 42 and 43, insertion parts 105a are formed near the ends of the shaft 105 in a flat-sided oval shape, and are non-rotatably inserted in flat-sided oval-shaped insertion holes 106a and 107a of the pressing members 106 and 107. Bearing supports 105b are formed at the ends of the insertion parts 105a of the shaft 105. The bearing supports 105b are rotatably supported in insertion holes 101d of the frame 101. Pressing springs 106b and 107b that press on the bent components 102c of the thermal head 102 are attached to the pressing members 106 and 107, respectively. As shown in FIG. 40, the pressing member 106 is disposed so as to engage with the small-diameter gear 108a of the drive gear 108. The drive gear 108 is attached to the first surface 101a of the frame 101, and transmits drive force from the intermediate gear 112 to the pressing member 106. The drive force of the motor 110 (see FIG. 41) attached to the motor bracket 111 is transmitted through the intermediate gear 112 (see FIG. 44) to the large-diameter gear 108b of the drive gear 108, then to the small-diameter gear 108a to the pressing member 106.
FIGS. 40 and 41 show how the thermal head 102 presses on the platen roller 103 with the above-mentioned conventional thermal transfer printer. The drive force of the motor 110 is transmitted through the intermediate gear 112 and the large-diameter gear 108b and the small-diameter gear 108a of the drive gear 108 to the pressing member 106, which causes the pressing member 106 to pivot while being supported in the insertion holes 101d of the frame 101. As a result, the pressing spring 106b of the pressing member 106 presses on the bent component 102c on the first surface 101a side of the frame 101. Since the pressing members 106 and 107 are non-rotatably attached to the shaft 105, pivoting of the pressing member 106 causes the shaft 105 and the pressing member 107 to pivot as well. As a result, the pressing spring 107b of the pressing member 107 presses on the bent component 102c on the second surface 101b side of the frame 101. Consequently, the head component 102b of the thermal head 102 is pressed against the platen roller 103 against the biasing force of the torsion coil spring 109.
In the conventional thermal transfer printer shown in FIGS. 40 to 44, the insertion portions 105a at the ends of the shaft 105 are formed in a flat-sided oval shape in order to non-rotatably link the shaft 105 and the pressing members 106 and 107, which are the components that press on the thermal head 102. Obtaining such oval shape requires a time-consuming cutting procedure. Furthermore, since the shaft diameter of the bearing supports 105b at the ends of the insertion parts 105a has to be made smaller than the width of the flat-sided portion of the insertion portions 105a, it is also necessary to perform a cutting procedure on the insertion portions 105a. Therefore, it takes a long time to manufacture the insertion portions 105a. 
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for improved thermal transfer printers and image forming apparatuses that overcome the problems of the conventional art. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.